One of the most important characteristics of a winter tyre is its grip on snow surfaces (i.e. surfaces completely covered with snow), and therefore its ability to generate adequate friction as it rolls along the snow surface.
The friction generated between a winter tyre and a snow surface is the sum of a digging force, produced by the tyre tread penetrating the snow cover, and a shearing force, produced by friction between the snow trapped in the tyre tread (in particular, in the tread grooves) and the snow cover.
To increase the shearing force, the amount of snow trapped inside the tyre tread grooves must be increased; and, to increase the digging force, the tread pattern must have lots of edges capable of creating pressure peaks on the snow surface to penetrate the snow cover.
To increase the amount of snow trapped inside the tread grooves of a winter tyre, it has been proposed to form zig-zag lateral surfaces on the blocks defining the grooves. By so doing, the snow inside the groove tends to adhere to the zig-zag lateral surfaces of the blocks, thus trapping more snow inside the groove.
Patent Application W003013881A1 proposes a winter tyre particularly suitable for snow surfaces, and in which the lateral surfaces of each tread block have triangular or arrow-shaped cavities, which vary in depth (are shallowest at the tip) and are significantly elongated towards the facing groove.
Though successful in trapping more snow inside the tread grooves of winter tyres, and so increasing the shearing force, the above solutions fail to also increase the digging force and so achieve a winter tyre of good overall performance on snow surfaces.